This invention relates to optical waveguides in which all cross-sectional components of the waveguide structure may have the same nominal index of refraction. More particularly, it concerns such optical waveguides in which the light propagating cross-sectional region of the waveguide is imparted a higher than nominal index of refraction due to compressive loading of that region by other components of the waveguide structure.
Optical fiber waveguides have been utilized for the propagation of single-mode or multi-mode light energy in communications networks. Such waveguides conventionally employ a core along which the major portion of light energy is propagated and an exterior cladding. The optical function of the cladding is primarily as a medium having a lower index of refraction than the core so that the propagated light is confined to the core by refraction. Materials conventionally used to form the core and cladding may be various types of glass but substantially pure fused silica is preferred. Where fused silica is used as both the core and the cladding, the relative indices of refraction are effected by doping either the core or the cladding or both with various dopants which may either increase or decrease the index of refraction of the doped glass or fused silica.
U.S. Pat. No. Re. 28,664 reissued Dec. 23, 1975 contains a disclosure exemplary of optical waveguide structures which take advantage of the relative indices of refraction in glass or fused silica and air. In other words, the index of refraction of pure fused silica is higher than that of air by an amount adequate to enable propagation of light energy along a core surrounded by little more than air. Waveguides of this type conventionally envelope the core in a tube of glass to protect the core against contamination by foreign material and also to mitigate the effect of external perturbations.
While the prior art as represented by the afore-mentioned reissue patent shows promise from the standpoint of simplifying the fabrication of optical waveguides, structures heretofore developed are vulnerable from the standpoint of strength and also from the standpoint of polarization locking. In this latter respect, it is important, particularly to achieve the full benefit of single mode light energy propagation, that the cross-sectional region through which the single mode is propagated have constant axes of polarization irrespective of external perturbations. Heretofore, this problem has been addressed in conventional waveguides by reliance on the cladding to effect a diametric loading stress on the core for developing stress birefringence for purposes of polarization locking. Such internal stressing of the light propagating region of single material optical waveguides has not been possible out of the relatively fragile construction of single material waveguides heretofore proposed.